(1) Field of the Invention
The present invention relates to the field of rotors having hinged blades for rotorcraft fitted with at least one main rotor providing lift and propulsion, and more particularly helicopters. The present invention relates more specifically to mechanisms fitted to such rotors in order to limit the flapping path of blades of a rotary wing constituted by the rotor.
(2) Description of Related Art
The present invention provides such a mechanism that uses abutment members that oppose movement of the blades in flapping. The mechanism is more particularly of the type in which the abutment members are drivable by opposing drive means between two positions. One of the drive means uses centrifugal force to cause the abutment members to be retracted, while the other drive means relies on a spontaneous return of the abutment members to their initial position for limiting the flapping path of the blades.
The present invention also provides a rotorcraft rotor constituting a rotary wing that has blades hinged on a hub of the rotor and that is fitted with such an abutment mechanism for limiting the flapping path of the blades.
Helicopters are rotorcraft that are fitted with at least one main rotor providing the helicopter with both lift and propulsion. The rotor under consideration for the present invention comprises in particular a rotary wing having a plurality of blades, each blade being individually hinged to a rotary hub, with the blades being radially distributed around the axis of rotation of the hub. The blades are generally planar elongate elements, each carried at one end by the hub so that they extend transversely relative to its axis of rotation. Hinged mounting of blades on the hub is obtained by intermediate mounting members, each presenting individual freedoms of movement for the blades relative to the hub.
The mounting members respectively fitted to each of the blades at their ends engaging the hub allow the blades to be controlled by an operator, in particular a pilot. Hinged mounting of blades on the hub allows the helicopter pilot in flight to cause the pitch of the blades to vary collectively and/or cyclically in order to control the behavior of the rotorcraft relative to its lift and/or its propulsion.
The blades are free to move on the hub upwards and downwards. The concept of up and down should be considered relative to the orientation of the axis of rotation of the rotor when installed in position on a helicopter. When the rotor is rotating at nominal speed, the blades are spontaneously moved upwards under the effect of centrifugal force so as to be perpendicular relative to the axis of rotation of the rotor. When the rotor is stationary, the blades are not subjected to any centrifugal force and they naturally droop downwards under the effect of their own weight.
It is necessary to hold the blades flat when the rotor is stationary and to prevent them from flapping in order to preserve them. Flapping of the blades should also be avoided while the rotor is starting. Such flapping of the blades can be caused by the effect of forces external to the helicopter, e.g. induced by strong wind or by gusting, or indeed by the movements of a ship having the helicopter on board.
The rotor is thus commonly fitted with an abutment mechanism that limits the flapping path of the blades under the effect of forces external to the helicopter. For each blade, the abutment mechanism comprises a low abutment member and a high abutment member, which members form obstacles on the individual flapping paths of the blades respectively downwards and upwards. The abutment members co-operate with corresponding contact members forming parts of the members for hinge-mounting of the blades on the hub.
When the rotor is used to procure lift and/or propulsion for a helicopter, the high abutment members must not constitute an obstacle for upward movement of the blades. Such movement of the blades must be allowed while the rotor is in operation in order to allow the pilot to operate the blades by varying their pitch cyclically or collectively.
It is therefore necessary to provide means for retracting the high abutment members while the rotor is in operation. Nevertheless, account must be taken of the advisability of limiting flapping movement of the blades during a rotor starting stage.
The high abutment members are also used for preventing the blades from moving in the event of the blades being folded sideways in order to facilitate transporting and/or storing the helicopter. It must be possible to fold the blades sideways without significant inconvenience, and conversely it must be possible to deploy the blades quickly in order to put the helicopter into operation.
Consequently, the abutment mechanism includes a device for blocking the high abutment members against corresponding contact surfaces included in the mounting members. The high abutment members are movable on the hub between two positions. A first position is an engagement position in which the high abutment members are engaged against the corresponding contact members for limiting the freedom of the blades to move in flapping while the rotor is stationary between the low abutment members and the high abutment member associated therewith. A second position is a position in which the high abutment members are disengaged, thereby allowing the blades to move freely in upward flapping.
The high abutment members are carried by the hub via movement means, e.g. comprising a ring that carries all of the high abutment members and that is mounted to turn about the hub. In another embodiment, the high abutment members may be mounted to be individually movable in pivoting on a collar at the periphery of the hub.
The blocking device includes opposing drive means for moving the high abutment members between said engagement and disengagement positions. The drive means are suitable for being engaged on a common member for moving the high abutment members together, e.g. via such a said ring. When the high abutment members are mounted to be individually movable on the hub, the drive means may for example likewise be individually associated with each of the high abutment members.
The centrifugal force due to the rotor being set into rotation is advantageously used to cause the high abutment members to be retracted. Making use of centrifugal force in this way serves in particular to obtain the advantage of being adapted to starting the rotor in a high wind or when the wind is gusty, while using an organization for the drive means that is simple.
Beyond a predetermined threshold of rotor rotation, the high abutment members are spontaneously driven into the disengagement position. Conversely, while stopping the rotor, the centrifugal force that was being used lessens and allows the high abutment members to go to the engagement position. In order to avoid the blades from flapping during the terminal stage of stopping the rotor, return means are used to force the high abutment members to pass into the engagement position.
Consequently, the drive means comprise two opposing drive means for driving the high abutment members, respectively to the disengagement position and to the engagement position.
First drive means for the high abutment members are of the centrifugal force type for causing the high abutment members to move spontaneously into the disengagement position under the effect of the centrifugal force caused by the rotor rotating. The first drive means typically make use of at least one flyweight that is carried by a lever arm engaged with the hub and with the high abutment members. The engagement of the lever arm with the high abutment members may be individual or collective, depending on the above-mentioned variant ways of organizing said movement means.
Second drive means for the high abutment members oppose the forces generated by the first drive means. The second drive means are advantageously of the spontaneous rated return means type for returning the high abutment members to the engagement position. The second drive means generate a spontaneous return of the high abutment members to the engagement position as soon as the centrifugal force used by the first drive means lessens below a predetermined threshold. The rated return means are typically elastically deformable means, such as a compression spring, a traction spring, and/or a torsion spring, for example.
While the rotor is being put into rotation, centrifugal force as a result of the rotation causes the first drive means to tend to entrain the high abutment members to the disengagement position. A return force exerted by the second drive means opposes premature passage of the high abutment members to the disengagement position, so long as the said corresponding force threshold is not reached.
Conversely, while the rotor is stopping, there is a lessening of the centrifugal force used by the first drive means. This lessening reduces the ability of the first drive means to oppose the return force generated by the second drive means. From the corresponding force threshold, the centrifugal force lessened by the slowing of the rotor allows the high abutment members to move towards the engagement position under the effect of the return force that has become greater than the centrifugal force. In the absence of the rotor being set once more into rotation, the return force keeps the high abutment members in the engagement position.
For information about a technological environment that is close to the present invention, reference may be made for example to the following documents: FR 2 885 115 (Eurocopter France); and FR 2 725 687 (Eurocopter France); which documents describe such abutment mechanisms for a helicopter rotor, suitable for providing a flapping limit for hinge-mounted blades on a hub of the rotor.
In the course of using such abutment mechanisms, it has been found that they could be improved, both in terms of their operation and in terms of preserving mechanical parts of the rotor.
A retraction stage is identified during which the high abutment members are retracted from their engagement position to their disengagement position. This retraction stage extends over a corresponding duration while the speed of rotation of the rotor is increasing on starting, until the high abutment members are indeed placed in the disengagement position. During the retraction stage, mechanical parts in contact with the abutment mechanism are subjected to harmful stresses or impacts that damage them, particularly in the presence of a high wind or of gusts of wind urging the blades upwards.
Such stresses arise more particularly during a transient step of the retraction stage, in which the high abutment members are set into movement in order to escape from the contact members that they engage in the engagement position. The transient step extends from the instant at which disengagement of the high abutment members is initiated, until the disengagement is complete.
It is desirable for the passage of the high abutment members from the engagement position to the disengagement position to be as fast as possible once their movement has been initiated.